My buns will chew anything rubber or foam or soft plastic. I had a gate to keep them contained once. A few nibbles and they were free.

Wood is good. I usually have cardboard houses for mine. They don't eat it as much as tear it. Either it's sticking time bomb or I'm lucky. But 3 years now and no blockages. I'm inclidlnded to say it's safe.

I so plan on making them some sort of plywood hutch to replace their very disposable box houses.

Do they have hay to eat? A big box or tray (eg underbed storage box) full will help to distract them from chewing things that they shouldn't. I would watch their droppings for a few days to make sure that everything is passing through OK - or they may need an urgent vet visit.

They have plenty of hay, which they don't eat near enough of. They don't eat the cardboard that much. Just tear it off. It's the only thing they chew. Actually surpised their teeth aren't getting too long. I think it's the cardboard. Be nice if they chewed on the pumice stone.